Clasificación de las cuencas hidrológicas

Ten percent aqueous suspension of pea starch was prepared at room temperature by stirring a mixture of starch and water at 950 rpm for 20 min using a magnetic stirrer. The sample dispersion unit of the Mastersizer 2000 (Malvern Instruments, Malvern, UK) was rinsed thoroughly before use. Mastersizer 2000 operates on the principle that a laser beam gets scattered by particles passing through it at angles and intensities inversely and directly proportional to their sizes, respectively (Malvern 2008). The starch suspension was fed in drops into the small sample volume dispersion unit until the laser obscuration was between 10 to 20% before running the particle size determination experiment. The equipment was set to perform two scans of the same sample and return their average.

The volumetric particle size distribution of the pea starch as obtained using laser diffraction technique is shown in Figure 3.1. The starch particles were predominantly between 10 and 100 µm in diameter with the average size being 31 µm.

3.2.3 Particle densities of pea starch, flax fiber, and polycaprolactone A nitrogen-operated multipycnometer (Quantachrome Instruments, Boynton Beach, FL) was used to determine the particle densities of both pea starch and PCL. Pea starch was used as is. A sample cell was first of all weighed empty, filled to about two-thirds of its volume with starch, flax fiber or PCL and re- weighed in order to obtain the sample mass. The procedure outlined in the

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multipycnometer manual was followed for the particle density determination.

Figure 3.1 Particle size distribution of pea starch using laser diffraction technique.

The operational equation as obtained from the equipment manual is as given below: P P

V

M

D

=

D = sample (pea starch or PCL) particle density, g/cm3

MP = sample (pea starch or PCL) mass, g VP = sample (pea starch or PCL) volume, cm3 and

)

1

)

((

−

−

=

P

P

V

V

V

60 where:

VP = volume of powder (pea starch or PCL), cm3 VC = volume of sample cell, cm3

VR = reference volume, cm3

P1 = pressure reading after pressurizing the reference volume, psi P2 = pressure reading after including VC, psi

The particle densities of pea starch, flax fiber, and PCL taken as average of three measurements were found to be 1.46 ± 0.02, 1.41 ± 0.02, and 1.15 g/cm3, respectively. This shows the starch particles were heavier than PCL particles.

3.2.4 Gelatinization properties of the supplied pea starch

Aqueous suspensions containing 17.45 and 26.17% by weight of the supplied pea starch were prepared with distilled water and left to hydrate for 12 h. Ten to 20 mg of the samples were placed in aluminum hermetic pans, crimped tightly to avoid vapour escape and then run through a DSC Q2000 (TA Instruments, New Castle, DE) from 20 to 150°C at a scan rate of 5°C/min under nitrogen atmosphere. Endothermic peaks attributable to gelatinization transition were obtained and the gelatinization properties such as onset, mid-point, and end- point gelatinization temperatures were obtained from the endotherms. The gelatinization enthalpy was obtained by finding the area under the graph within the gelatinization transition boundaries.

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The gelatinization transitions obtained by the DSC are shown in Figure 3.2. The DSC thermograms show endotherms typical of starch gelatinization in excess water (Stevens and Elton 1971; Wooton and Bamunuarachchi 1980; Donovan 1979; Eliasson 1980; Fukuoka et al. 2002; Yu and Christie 2001). From Figure 3.2a, pea starch onset, peak (mid-point), and end of gelatinization temperatures were found to be 57.60°C, 68.24°C, and 80.33°C, respectively. The enthalpy of gelatinization obtained as the area under the curve between the transition limits was 2.87 J/g.

It is however important to emphasize that starch gelatinization is a function of water and temperature (Fukuoka et al. 2002). This is evident in the presence of two endothermic peaks when the starch (dry mass) concentration was increased to 26.17% as shown in Figure 3.2b. The enthalpies of gelatinization obtained from the first and second endotherms were 1.24 J/g and 5.65 J/g, respectively. Although there was a decrease in the specific enthalpy of gelatinization from 2.87 to 1.24 J/g with decrease in moisture content, the onset, mid-point, and end point gelatinization temperatures remained fairly the same for the common first endothermic peak. These results are in agreement with Fukuoka and fellow workers’ (2002) observation.

Possibilities of three endothermic peaks have been reported in limited water conditions (Fukuoka et al. 2002; Donovan 1979). The first and second (which unlike the first shifts further to the right with decrease in moisture content) endothermic peaks are due to moisture-induced breakdown of starch crystallites and subsequent “melting” of remnant crystallites, respectively

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(a) 17.45% aqueous suspension of pea starch

(b) 26.17% aqueous suspension of pea starch

Figure 3.2 Effects of moisture content on the gelatinization of pea starch using differential scanning calorimetry technique.

Heat Flow (W/g) 0 20 40 60 80 100 120 140 160 Temperature (°C) -0.15 -0.20 -0.25 -0.30 -0.35 -0.40 Exo Up 56.85°C 74.32°C 67.31°C 101.33°C 109.72°C 105.94°C 57.60°C 80.33°C 68.24°C 0 20 40 60 80 100 120 140 160 Temperature (°C) 0.1 0.0 -0.1 -0.2 -0.3 -0.4 Heat Flow (W/g) Exo Up

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(Donovan 1979; Fukuoka et al. 2002). The order-disorder transition of amylose- lipid complexes is essentially believed to be responsible for the third peak (Biliaderis 1990). By and large, the first peak is usually analyzed for gelatinization parameters (Fukuoka et al. 2002).

3.2.5 Thermal behavior of the supplied PCL and flax fiber

PCL powder of 11.81 mg was placed in hermetic aluminum pans, crimped tightly, and run through the DSC Q2000. A heat-cool-heat method was employed. The sample was heated to 100°C and cooled to -80°C at a scan rate of 15°C/min, and then heated to 400°C at 10°C/min under nitrogen atmosphere. The first heating cycle was to erase the thermal history of the sample (Wielage et al. 1999) and therefore, only the second heating cycle was used for analytical purposes. Similarly, 4.38 mg of washed flax fiber was placed in aluminum hermetic pan and run through the DSC from -80 to 400°C under nitrogen atmosphere at a scan rate of 10°C/min to determine its thermal behavior.

The DSC thermogram obtained for PCL is as shown in Figure 3.3a. PCL is known to undergo three thermal transitions when heated. These are glass transition, melting, and thermal decomposition. As shown in Figure 3.3, the glass transition and melting temperatures were found to be -62.63°C and 58.96°C (Appendix A) while at about 400°C, the material exhibited thermal decomposition. Similar results have been obtained in previous studies (Chrissafis et al. 2007; Polo Fonseca et al. 2007; Shin et al. 2004; Sarazin et al. 2008). Figure 3.3b shows the thermal behavior of flax fiber using DSC. The

64 (a)

(b)

Figure 3.3 Differential scanning calorimetry thermograms of (a) polycaprolactone; (b) flax fiber.

-62.63°C 58.96°C 0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -100 0 100 200 300 400 Temperature (°C) Heat Flow (W/g) Exo Up 0.0 -0.2 -0.4 -0.6 -0.8 Heat Flow (W/g) Exo Up 91.12°C 220.14°C -100 0 100 200 300 400 Temperature (°C)

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broad peak could be attributed to moisture vaporization from the fiber which is then followed by thermal degradation beyond 350°C. Similar thermal degradation of flax fiber has been reported using DSC and thermogravimetric analysis (TGA) in previous studies (Manfredi et al. 2006; Wielage et al. 1999).